NHibernate 2.0.1 下实现SQL2000真分页

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最近用NHibernate做个项目 由于数据库用的是mssql2000 NHibernate 对mssql2000的分页查询支持的不是很好 于是自己动手实现一个mssql2000方言

原 NHibernate.Dialect 命名空间下的mssql2000的方言类 MsSql2000Dialect 里的 GetLimitString 方法 如下：

Code
public override SqlString GetLimitString(SqlString querySqlString, int offset, int limit)
{
if (offset > 0)
{
throw new NotSupportedException("SQL Server does not support an offset");
}

/*
* "SELECT TOP limit rest-of-sql-statement"
*/

return querySqlString.Insert(GetAfterSelectInsertPoint(querySqlString), " top " + limit.ToString());
}



看上面这段代码大家都知道这种分页方式的性能有多么的差 下面我把我自己实现的方言类代码贴出来



Code
public class mysqlServer2000Dialect : MsSql2000Dialect
{
private static int GetFromIndex(SqlString querySqlString)
{
string subselect = querySqlString.GetSubselectString().ToString();
int fromIndex = querySqlString.IndexOfCaseInsensitive(subselect);
if (fromIndex == -1)
{
fromIndex = querySqlString.ToString().ToLowerInvariant().IndexOf(subselect.ToLowerInvariant());
}
return fromIndex;
}


private static string RemoveSortOrderDirection(string sortExpression)
{
// Drop the ASC/DESC at the end of the sort expression which might look like "count(distinct frog.Id)desc" or "frog.Name asc".
return Regex.Replace(sortExpression.Trim(), @"(\)|\s)(?i:asc|desc)$", "$1").Trim();
}

public override bool SupportsLimitOffset
{
get
{
return true;
}
}

public override NHibernate.SqlCommand.SqlString GetLimitString(SqlString querySqlString, int offset, int limit)
{
if (offset == 0)
{
return base.GetLimitString(querySqlString, offset, limit);
}

SqlString myQuery = new SqlString(RemoveSortOrderDirection(querySqlString.ToString().ToLower()));

int orderIndex = myQuery.LastIndexOfCaseInsensitive(" order by ");

if (orderIndex <= 0)
{
throw new NotSupportedException("must specify 'order by' statement to support limit operation with offset in SqlServer2000");
}

string orderBy = myQuery.Substring(orderIndex).ToString().Trim();

string[] sortExpressions = myQuery.Substring(orderIndex).ToString().Trim().Substring(9).Split(',');

int fromIndex = GetFromIndex(myQuery);

SqlString from = myQuery.Substring(fromIndex, orderIndex - fromIndex).Trim();

string fromAsName = from.ToString().Substring(from.ToString().IndexOf(' ', 5)).Trim();

SqlString select = myQuery.Substring(0, fromIndex);

int PageSize = limit - offset;

SqlStringBuilder result = new SqlStringBuilder();

result.Add("declare @indextable table(id int identity(1,1),nid int) set rowcount " + limit.ToString());

result.Add(" insert into @indextable(nid) select");

result.Add(" " + RemoveSortOrderDirection(sortExpressions[0]).Replace(fromAsName,"t") + " " + from.ToString().Replace(fromAsName,"t") + " " + orderBy.Replace(fromAsName,"t") + " desc ");

result.Add(" " + select.ToString() + " " + from.ToString() + ",@indextable a where " + RemoveSortOrderDirection(sortExpressions[0]) + " = a.nid and a.[id] between " + (offset + 1).ToString() + " and " + limit.ToString());

return result.ToSqlString();
}
}
这个类继承自原 NHibernate 的 mssql2000方言类 重写了 SupportsLimitOffset 属性 返回 true 让 NHibernate 支持 offset

重写了 GetLimitString 方法 使用 内存变量表的方式进行数据分页 前提是 hql 语句里必须带有主键排序字段 主键必须是 自动增长

下面介绍如何使用这个类来替换 NHibernate 下的原mssql2000方言：

　　　　在NHibernate配置文件里的找到 <property name="dialect">NHibernate.Dialect.MsSql2000Dialect</property>

　　　　替换成 <property name="dialect">myMsSql2000Dialect类的命名空间.myMsSql2000Dialect</property>

　　　 哪位高人有更好的实现方式的话 别忘了共享给我下